Multimode planar waveguide spectral filter

ABSTRACT

A spectral filter comprises a planar optical waveguide having at least one set of diffractive elements. The waveguide confines in one transverse dimension an optical signal propagating in two other dimensions therein. The waveguide supports multiple transverse modes. Each diffractive element set routes, between input and output ports, a diffracted portion of the optical signal propagating in the planar waveguide and diffracted by the diffractive elements. The diffracted portion of the optical signal reaches the output port as a superposition of multiple transverse modes. A multimode optical source may launch the optical signal into the planar waveguide, through the corresponding input optical port, as a superposition of multiple transverse modes. A multimode output waveguide may receive, through the output port, the diffracted portion of the optical signal. Multiple diffractive element sets may route corresponding diffracted portions of optical signal between one or more corresponding input and output ports.

RELATED APPLICATIONS

[0001] This application claims benefit of prior-filed co-pendingprovisional App. No. 60/486,451 entitled “Focusing holographic elementsin multimode waveguides” filed Jul. 10, 2003 in the names of Thomas W.Mossberg, Christoph M. Greiner, and Dmitri Iazikov, said provisionalapplication being hereby incorporated by reference as if fully set forthherein.

[0002] This application is a continuation-in-part of prior-filedco-pending U.S. non-provisional App. No. 10/653,876 entitled “Amplitudeand phase control in distributed optical structures” filed Sep. 2, 2003in the names of Christoph M. Greiner, Dmitri Iazikov, and Thomas W.Mossberg, which is in turn a continuation-in-part of U.S.non-provisional App. No. 10/229,444 entitled “Amplitude and phasecontrol in distributed optical structures” filed Aug. 27, 2002 in thenames of Thomas W. Mossberg and Christoph M. Greiner, now U.S. Pat. No.6,678,429 issued Jan. 13, 2004. Each of said application and said patentare hereby incorporated by reference as if fully set forth herein. App.No. 10/229,444 in turn claims benefit of provisional App. No. 60/315,302entitled “Effective gray scale in lithographically scribed planarholographic devices” filed Aug. 27, 2001 in the name of Thomas W.Mossberg, and provisional App. No. 60/370,182 entitled “Amplitude andphase controlled diffractive elements” filed Apr. 4, 2002 in the namesof Thomas W. Mossberg and Christoph M. Greiner, both of said provisionalapplications being hereby incorporated by reference as if fully setforth herein.

[0003] This application is a continuation-in-part of prior-filedco-pending non-provisional App. No. 09/811,081 entitled “Holographicspectral filter” filed Mar. 16, 2001 in the name of Thomas W. Mossberg,and a continuation-in-part of prior-filed co-pending non-provisionalApp. No. 09/843,597 entitled “Optical processor” filed Mar. 16, 2001 inthe name of Thomas W. Mossberg, application Ser. No. 09/843,597 in turnbeing a continuation-in-part of said application Ser. No. 09/811,081.Said application Ser. No. 09/811,081 in turn claims benefit of: 1)provisional App. No. 60/190,126 filed Mar. 16, 2000; 2) provisional App.No. 60/199,790 filed Apr. 26, 2000; 3) provisional App. No. 60/235,330filed Sep. 26, 2000; and 4) provisional App. No. 60/247,231 filed Nov.10, 2000. Each of said non-provisional applications and each of saidprovisional applications are hereby incorporated by reference as iffully set forth herein.

BACKGROUND

[0004] The field of the present invention relates to optical devicesincorporating distributed optical structures. In particular, multimodeplanar waveguide spectral filters are disclosed herein.

SUMMARY

[0005] A spectral filter comprises a planar optical waveguide having atleast one set of diffractive elements. The planar optical waveguidesubstantially confines in one transverse spatial dimension an opticalsignal propagating in two other spatial dimensions therein. The planarwaveguide supports multiple optical transverse modes in the confinedtransverse dimension. Each diffractive element set routes, between acorresponding input optical port and a corresponding output opticalport, a corresponding diffracted portion of the optical signalpropagating in the planar waveguide that is diffracted by thediffractive element set. The corresponding diffracted portion of theoptical signal reaches the corresponding output optical port as asuperposition of multiple optical transverse modes supported by theplanar optical waveguide. A corresponding multimode optical source maybe positioned and aligned so as to launch the optical signal into theplanar waveguide, through the corresponding input optical port, as asuperposition of multiple optical transverse modes supported by theplanar optical waveguide. A corresponding multimode output waveguide maybe positioned and aligned so as to receive, through the correspondingoutput optical port, the corresponding diffracted portion of the opticalsignal. Multiple diffractive element sets may route correspondingdiffracted portions of optical signal between one or more correspondinginput optical ports and one or more corresponding output optical ports.

[0006] Objects and advantages pertaining to multimode planar waveguidespectral filters may become apparent upon referring to the disclosedembodiments as illustrated in the drawings and disclosed in thefollowing written description and/or claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIGS. 1A and 1B are side and top schematic views, respectively, ofa planar waveguide spectral filter. FIG. 1C is a schematic top viewshowing an optical path between input and output optical ports of aplanar waveguide spectral filter.

[0008]FIGS. 2A and 2B are side schematic views of planar waveguidespectral filters.

[0009]FIG. 3 is a side schematic view of a planar waveguide spectralfilter.

[0010]FIG. 4 illustrates the dependence of mode angle on planarwaveguide thickness parameter.

[0011]FIG. 5 illustrates the dependence of fractional bandpass oncladding index for a planar waveguide spectral filter.

[0012] The embodiments shown in the Figures are exemplary, and shouldnot be construed as limiting the scope of the present disclosure and/orappended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

[0013] A spectral filter according to the present disclosure comprises amultimode planar optical waveguide having at least one set ofdiffractive elements. The planar optical waveguide substantiallyconfines in one transverse dimension optical signals propagating in theother two spatial dimensions. The planar waveguide supports multipletransverse optical modes in the confined transverse dimension. Anoptical signal propagates through the multimode planar optical waveguideas a superposition of the supported optical modes. However, the relativephases of the modes of such a superposition generally vary withpropagation distance through the waveguide, since the optical modespropagate at differing phase velocities (i.e., the waveguide exhibitsmodal dispersion). The planar waveguide typically comprises a core (atwo-dimensional sheet or layer) surrounded by lower-index cladding. Thecore is fabricated using one or more dielectric materials substantiallytransparent over a desired operating wavelength range. In some instancesone or both claddings may be vacuum, air, or other ambient atmosphere.More typically, one or both claddings comprise layers of dielectricmaterial(s), with the cladding refractive indices n₁ and n₂ typicallybeing smaller than the core refractive index n_(core). (In someinstances in which short optical paths are employed and some degree ofoptical loss can be tolerated, the cladding indices might be larger thanthe core index while still enabling the planar waveguide to supportguided, albeit lossy, optical modes.) The planar waveguide may besecured to a substrate, for facilitating manufacture, for mechanicalsupport, and/or for other reasons.

[0014] The set of diffractive elements of the spectral filter may alsobe referred to as: a set of holographic elements; a volume hologram; adistributed reflective element, distributed reflector, or distributedBragg reflector (DBR); a Bragg reflective grating (BRG); a holographicBragg reflector (HBR); a directional photonic-bandgap structure; amode-selective photonic crystal; or other equivalent terms of art. Eachdiffractive element of the set diffracts, reflects, scatters, orotherwise redirects a portion of an incident optical signal (saidprocess hereinafter simply referred to as diffraction). Each diffractiveelement of the set typically comprises some suitable alteration of theplanar waveguide (ridge, groove, index modulation, density modulation,and so on), and is spatially defined by a virtual two-dimensionalcurvilinear diffractive element contour, the curvilinear shape of thecontour typically being configured to impart desired spatialcharacteristics onto the diffracted portion of the optical signal. Thecurvilinear diffractive elements of the set (or equivalently, theircorresponding contours) are spatially arranged with respect to oneanother so that the corresponding portions of the optical signaldiffracted by each element interfere with one another, so as to impartdesired spectral and/or temporal characteristics onto the portion of theoptical signal collectively diffracted from the set of diffractiveelements. The diffractive elements in the set are arranged so that aninput optical signal, entering the planar waveguide through an inputoptical port, is successively incident on diffractive elements of theset. A fraction of the incident amplitude is diffracted by a diffractiveelement and the remainder transmitted and incident on anotherdiffractive element, and so on successively through the set ofdiffractive elements. The diffractive elements are therefore spacedsubstantially longitudinally along the propagation direction of theincident optical signal (in contrast to a traditional surface grating,in which the diffractive elements, i.e. grating lines, are spacedtransversely across the wavefront of a normally incident opticalsignal). Each curvilinear diffractive element is shaped to direct itsdiffracted portion of the optical signal to an output optical port,typically propagating back through earlier diffractive elements of theset. The relative spatial arrangement (i.e. longitudinal spacing) of thediffractive elements of the set yields desired spectral and/or temporalcharacteristics for the overall diffracted optical signal reaching theoutput optical port. It should be noted that optical ports (input and/oroutput) may be defined structurally (for example, by an aperture,waveguide, fiber, lens, or other optical component) and/or functionally(i.e., by a spatial location, convergence/divergence/collimation, and/orpropagation direction). For a single-mode planar waveguide, such a setof diffractive elements may be arranged to yield an arbitrary spectraltransfer function (in terms of amplitude and phase). In a multimodeplanar waveguide, the subject of the present disclosure, modaldispersion and mode-to-mode coupling of diffracted portions of theoptical signal limit the range of spectral transfer functions that maybe implemented.

[0015] The set of diffractive elements of the spectral filter providesdual functionality, spatially routing an optical signal between an inputoptical port and an output optical port, while at the same time actingas a spectral filter. The curvilinear diffractive elements may bedesigned (by computer generation, for example) so as to provide optimalrouting, imaging, or focusing of the optical signal between an inputoptical port and a desired output optical port, thus reducing orminimizing insertion loss of the spectral filter. Simple curvilineardiffractive elements (segments of circles, ellipses, parabolas,hyperbolas, and so forth), if not optimal, may be employed asapproximations of fully optimized contours. A wide range of fabricationtechniques may be employed for forming the diffractive element set, andany suitable technique(s) may be employed while remaining within thescope of the present disclosure and/or appended claims. The followingare exemplary only, and are not intended to be exhaustive.

[0016] Diffractive elements may be formed lithographically on thesurface of a multimode planar optical waveguide, or at one or bothinterfaces between core and cladding of a multimode planar opticalwaveguide. Diffractive contours may be formed lithographically in theinterior of the core layer and/or a cladding layer of the planar opticalwaveguide using one or more spatial lithography steps performed after aninitial partial deposition of layer material. Diffractive elements maybe formed in the core and/or cladding layers by projecting ultravioletlight or other suitable radiation through an amplitude and/or phase maskso as to create an interference pattern within the multimode planarwaveguide (fabricated at least in part with suitably sensitive material)whose fringe contours match the desired diffractive element contours.Alteration of the refractive index by exposure to ultraviolet or otherradiation results in index-modulated diffractive elements. The mask maybe zeroth-order-suppressed according to methods known in the art,including the arts associated with fabrication of fiber Bragg gratings.The amplitude and/or phase mask may be produced lithographically vialaser writer or e-beam, it may be interferometrically formed, or it maybe formed by any other suitable technique. In instances where resolutionis insufficient to produce a mask having required feature sizes, alarger scale mask may be produced and reduced to needed dimensions viaphotoreduction lithography, as in a stepper, to produce a mask at theneeded scale. Diffractive elements may be formed by molding, stamping,impressing, embossing, or other mechanical processes. A phase mask maybe stamped onto the core or cladding surface followed by opticalexposure to create diffractive elements throughout the core and orcladding region. The optical or UV source used to write the diffractiveelements in this case should have a coherence length comparable orlonger than the distance from the stamped phase mask to the bottom ofthe core region. Stamping of the phase mask directly on the device maysimplify alignment of diffractive elements with ports or other devicecomponents especially when those components may be formed in the same oranother stamping process. Many approaches to the creation of refractiveindex modulations or gratings are known in the art and may be employedin the fabrication of diffractive element sets.

[0017] Irradiation-produced refractive index modulations or variationsfor forming diffractive elements will optimally fall in a range betweenabout 10⁻⁴ and about 10⁻¹; however, refractive index modulations orvariations outside this range may be employed as well. Refractive indexmodulations or variations may be introduced by light of any wavelength(including ultraviolet light) that produces the desired refractive indexchanges, provided only that the photosensitive material employed issuitably stable in the presence of light in the desired operatingwavelength range of the spectral filter. Exposure of a complete set ofdiffractive elements to substantially spatially uniform,refractive-index-changing light may be employed to tune the operativewavelength range of the diffractive element set. Exposure of thediffractive element set to spatially non-uniform refractive-indexchanging light may be employed to chirp or otherwise wavelength-modulatethe spectral filter (described further hereinbelow). The sensitivity ofplanar waveguide materials to irradiation produced refractive indexmodulations may be increased using hydrogen-loading, flame-brushing,boron or other chemical doping, or other method known in the art, forexample in the context of making fiber Bragg gratings.

[0018] A schematic overview of an exemplary planar waveguide spectralfilter is shown in FIG. 1. In FIG. 1A, a cross section of a planarwaveguide 100 (with core 104 and first and second claddings 106 and 108)shows diffractive elements 102 in cross section. In this exemplaryembodiment, the diffractive elements are formed at the interface betweencore 104 and cladding 106, and comprise cladding material deposited intotrenches etched to a depth d into the one surface of the waveguide corelayer 104. The diffractive elements 102 may instead be filled with amaterial other than the cladding material, provided the material has arefractive index differing from the core refractive index. An opticalsignal propagates in the x- and z-dimensions through the planarwaveguide (substantially confined in the y-dimension by the planarwaveguide) and is backscattered by the diffractive elements. Light whosevacuum wavelength λ_(vac)=2n_(eff)Λ will be most strongly backscattered(due to constructive interference among the portions of the incidentoptical signal diffracted by the individual diffractive elements). Heren_(eff) is the effective waveguide refractive index and Λ is thephysical spacing between diffractive contours (defined herein in termsof the differences in the lengths of ray paths s_(arc) between input andoutput ports for successive diffractive elements, as shown in FIG. 1C;the spacing may be averaged over a diffractive element if the raypathlength is not constant across the diffractive element, i.e., if thediffractive element curvilinear contour is not fully spatiallyoptimized). For example, if λ_(vac)=1.5 μm is to be reflected and theeffective core refractive index for the mode of interest is 1.45, theone-way ray path spacing between diffractive elements should be about0.517 μm.

[0019] It should be noted that the effective waveguide index is notsimply the refractive index n_(core) of the bulk material comprising thecore. The effective core index reflects rather the property of aparticular mode supported by the planar waveguide and represents thevacuum speed of light divided by the phase velocity of the guided wavealong the guiding direction of the waveguide. In a single-modewaveguide, there is a unique effective index for light traveling withinthe waveguide (typically wavelength-dependent). For a multimodewaveguide, the subject of the present disclosure, there is a range ofeffective waveguide refractive indices—one for each of the transverseoptical modes that is supported by the waveguide. In the context ofplanar waveguides, “transverse optical mode” refers to optical modeshaving distinct electric field (E-field) amplitude variations along thedirection locally normal to the plane of the planar waveguide. Eachtransverse optical mode actually comprises a family of modes having thesame E-field variation along the local normal to the planar waveguidebut having different propagation directions within the waveguide. Theeffective index for a particular optical mode is a function of the modesize and shape, the core index n_(core), the first and second claddingindices n₁ and n₂, and the core thickness D (note these are not allindependent of one another; the thickness and indices typicallydetermine the mode size and shape). The refractive indices of the twocladdings, n₁ and n₂, need not be the same. The cladding index closer tothe core index (i.e., the greater of n₁ and n₂) shall be denotedhereinafter as n_(clad). Note that air, dielectric, metallic, or othersuitable cladding material may be employed. Core and cladding materialsshould be chosen to provide acceptably low absorptive and scatteringlosses. Many appropriate silica, semiconductor, polymer, and othermaterials are known in the planar waveguide art and may be employedwithin the scope of the present disclosure and/or appended claims.

[0020] In FIG. 1B, a top view of the diffractive element set is shown.Each dashed curve represents a diffractive element 102 (equivalently, adiffractive element virtual contour). In this example the diffractiveelements are spatially defined by diffractive contours that areconcentric circular arcs (with common center of curvature 110). Eachdiffractive element acts as a weak focusing mirror that images the inputoptical port 112 onto the output optical port 114, with the portslocated at conjugate image points defined by the circular diffractiveelement contours. In a fully spatially optimized device, eachdiffractive element may be defined by a unique virtual contour designedso as to convert the input optical signal wavefront into an outputoptical signal wavefront matched as well as possible for effectiveoptical coupling into the output optical port. Simpler curvilineardiffractive element contours (such as circular arcs in this example) maybe employed as approximations of fully optimized contours. Opticalcoupling may be optimized by designing the diffractive element set toreside within and fill at least a portion of the region of overlapbetween the designed input optical signal beam and the designed outputoptical signal beam. The design output beam may be visualized byinjecting a signal backwards through the output port so as to impinge onthe diffractive element set. The time-reverse of the backward injectedsignal is the design output beam. The design input signal beamcorresponds to a beam launched toward the diffractive element setthrough the input port. When the input and output ports are in closeproximity and the design input and output beams are nearlyanti-parallel, the overlap region will be of large extent along a linebisecting the angle between input and output beams. The diffractiveelement set need not fill this entire length. Along this bisectordirection, the diffractive element set need only be long enough toprovide desired spectral resolution or overall reflective strength. In asingle mode waveguide, the fully optimized focusing contour is unique asthere is only one mode involved. In a multimode waveguide (the subjectof the present disclosure), coupling by the diffractive element set ofall possible input modes to all of the possible output modes isrelevant. FIG. 1C shows a particular ray path from the input opticalport 112 to the output optical port 114. Typically, the spacing betweendiffractive contours is arranged so that optical ray paths like the oneshown in FIG. 1C increment by a constant amount from one diffractivecontour to the next. (As an aside, since for a specific optical mode theoptical pathlength difference between diffractive elements depends onthe relevant modal indices as well as the ray pathlength differencebetween the diffractive elements, and since the modal indices vary amongthe supported transverse modes, the optical path increment cannot bedefined uniquely in a multimode waveguide. Therefore, when referringherein to a multimode waveguide, phrases such as “optical pathdifference” and so forth must be defined in a meaningful and consistentfashion. One such definition may assume the effective index for thelowest-order incident and diffracted modes. Another suitable definitionmay assume an average effective index for all supported modes. Yetanother definition may assume the bulk core index n_(core). Othersuitable definitions may be used as well, and any suitable definitionthat reasonably approximates the modal indices, consistently applied,shall fall within the scope of the present disclosure and/or appendedclaims.) This constant optical ray path increment does not, however,necessarily result in uniform physical spacing between the diffractiveelement contours. In the exemplary embodiment of FIG. 1A-1C, diffractivecontour spacings measured along the radial direction in FIGS. 1B and 1Care not constant, but instead are weakly chirped to yield a uniformincrement in the optical ray path length. It is possible to vary theoptical ray path increment to control the relative phase of signalsdiffracted by successive contours. Such control is useful for theintroduction of various types of apodization.

[0021] In FIG. 2A, a structure is shown wherein the diffractive elements102 are located in the interior of the core 104. Such interior-locateddiffractive elements may more strongly interact with lower ordertransverse modes. In general, larger fractions of the transverseprofiles of relevant transverse modes spanned by the diffractiveelements, whether positioned at the core/cladding interface, within thecore layer, and/or within the cladding layer, result in acorrespondingly larger fraction of the input optical signal diffractedby the diffractive elements (for a given index contrast between the coreand the diffractive elements). This in turn enables a desired level ofoptical power reflectivity by the diffractive element set using fewerdiffractive elements. In FIG. 2B, the diffractive elements 102 span theentire thickness of the core layer 104. Diffractive elements spanninglarger fractions of the transverse profiles of the relevant modes of theoptical signal field would also tend to reduce variation in diffractionstrength among the multiple transverse optical modes supported by theplanar optical waveguide.

[0022] In FIG. 3 is shown the schematic geometry of light backscatteringfrom diffractive elements within a multimode planar waveguide. Thediffractive element geometry shown (located at the core/claddinginterface) is simplified to facilitate definition of incidence anglesfor a representative input mode and a representative back-diffractedmode, but the following would apply to any diffractive element geometry.Each transverse optical mode within a multimode planar optical waveguidemay be thought of as a superposition of two plane waves, each of whichmakes an angle θ relative to the local plane of the waveguide (referredto as a propagation angle for the optical mode in question). Differenttransverse optical modes typically have different propagation angles θ.The propagation angles θ_(in) and θ_(out) shown in FIG. 3 represent,respectively, the angles made by the constituent plane wavesubcomponents of input and back diffracted optical modes of thewaveguide. Note that the input angle, θ_(in), and the back-diffractedoutput angle, θ_(out), are not shown as being equal. In general, thediffractive element set can back-diffract a specific input optical modeinto the same counter-propagating optical mode or into one or more ofthe other counter-propagating optical modes of the planar waveguide. Therelative strength of diffraction into the same or different transverseoptical modes depends on the geometry of the diffractive elements andtheir placement relative to the core layer 104.

[0023] Each input-mode-to-output-mode diffraction process will occurmost strongly at a unique vacuum wavelength. Diffraction will occurstrongly at wavelengths satisfying the following equation

λ/n _(core)=Λ(cos θ_(in)+cos θ_(out))m  (1)

[0024] where m=1, 2, 3, . . . is the diffractive order. The angles inEq. 1 are, as described above, the characteristic modal propagationangles for the incident and diffracted optical modes, respectively. InFIG. 4, we plot the characteristic modal propagation angles for allmodes that exist in a planar guide as a function of the effective guidethickness n_(core)d/λ, where d is the physical waveguide thickness asshown in FIG. 1. To calculate FIG. 4, a core index n_(core)=1.45 and acladding index n_(clad) =1.43 are assumed. As one moves to the right inFIG. 4, the waveguide core is getting thicker. Additional optical modesare supported as the core thickness increases. As each successiveoptical mode “turns on”, its characteristic incidence angle begins nearthe critical angle of the core-cladding interface and then decreases asthe waveguide becomes thicker. As the mode number increases, the modesbecome relatively evenly spaced throughout the angular range from 0° tothe critical angle. Diffraction may occur from an input optical signalin any supported optical mode to a back-diffracted output optical fieldin any supported optical mode. If there are N supported modes, there areN² potential scattering processes—some of which may be degenerate.

[0025] The maximum back-diffracted wavelength occurs for thelowest-order optical mode diffracted back into the lowest-order opticalmode. The minimum back-diffracted wavelength occurs for thehighest-order support optical mode diffracted back into thehighest-order supported optical mode. The interval between the maximumand minimum diffracted wavelengths, Δλ, can be approximated by

Δλ=2Λn _(core)(1−cos θ_(H))/m  (2)

[0026] where we have approximated the propagation angle of thelowest-order optical mode to be 0° and the propagation angle of thehighest-order supported optical mode to be θ_(H)=π/2−sin⁻¹(n_(clad)/n_(core)). In many cases of interest, the diffracted bandpasswidth, Δλ, is quite small relative to the diffracted vacuum wavelengthλ. In such cases, we can approximate the diffracted vacuum wavelength asλ=2n_(core)Λ/m and calculate a fractional filter bandpass ratio

Δλ/λ=(n _(core) −n _(clad))/n _(core)  (3)

[0027] where we have inserted the definition of θ_(H) into Eq. 2 toyield Eq. 3. The fractional bandpass for the portion of the opticalsignal diffracted by the diffractive element set of the multimodespectral filter is found to be simply related, via Eq. 3, to therefractive index difference between the core and cladding. A moreaccurate value of Δλ/λ for specific waveguide parameters can becalculated by standard methods well known in the art, but for manydesign purposes, Eq. 3 suffices.

[0028] In FIG. 5, the fractional bandpass of an exemplary multimodeplanar waveguide spectral filter with the core index 1.45 is shown as afunction of cladding index. The fractional bandpass is approximatelyindependent of the thickness of the waveguide core in the multipletransverse optical mode regime. It should be noted again that thewaveguide need not necessarily have symmetric claddings. The fractionalbandpass is essentially determined by the larger cladding index (i.e.,the cladding index exhibiting the smaller index contrast with the coreindex). It may be desirable to have a relatively large refractive indexdifferential when diffractive elements are formed from cladding materialat a core-cladding interface, in order to increase diffraction strengthor for other reasons. As long as one of the core-cladding interfaces hasa sufficiently low refractive index contrast, the range of guided modeswill remain small and Eq. 3 will typically suffice. An important designconsideration for a multimode planar waveguide spectral filter is thedegree to which the supported planar waveguide optical modes match themodes of input and output optical signal fields, for example opticalmodes supported by multimode optical fiber. The value of n_(clad) notonly affects the spectral filter bandwidth but also spatial modematching to input/output optical signal fields. Minimal insertion losseswill tend to occur when the respective angular spreads of the transversemodes supported by the planar optical waveguide and by input/outputcomponents (such as a multimode fiber, a multimode channel waveguide, anLED, a laser diode, and so forth) are substantially similar to oneanother.

[0029] One application for a multimode planar waveguide spectral filteris use as coarse WDM (wavelength division multiplexing) multiplexers anddemultiplexers. In that application area, bandpass widths of about 10 nmare typically employed. With a 10-nm bandpass width and an operatingwavelength of 1.5 microns, a minimal core-cladding index contrast ofabout 0.67 percent is required. Such an index contrast is readilyachieved with many materials well known in the art.

[0030] It should be noted that Δλ as given by Eqs. (2) and (3) is awavelength range for the center wavelengths of individual mode-to-modediffracted spectral peaks. A multimode planar waveguide spectral filterwith constant optical path spacing among its diffractive elements willexhibit within this overall bandwidth Δλ multiple discrete diffractedspectral peaks corresponding to the diffraction of the various forwardoptical modes into the various backward optical modes. If thesemode-to-mode diffracted spectral peaks are narrower than Δλ, then Δλ isa good approximation of the overall bandwidth for the portion of theoptical signal diffracted by the spectral filter (referred to asΔλ_(filter)). If, on the other hand, the mode-to-mode diffractedspectral peaks are broader than Δλ, then the overall spectral widthΔλ_(filter) is at least as broad as the broadest mode-to-mode diffractedspectral peak. If the diffractive element set has substantially constantoptical path spacing along its operative optical path and thediffractive element set is on the 1-cm scale, these individualmode-to-mode diffracted spectral peaks will be on the order of 10 GHzwide (for a core refractive index of about 1.5 and if the diffractiveelement set is only weakly, i.e. less than about 70%, reflective). Inthe low reflective strength limit, the individual spectral peaks broadenin inverse proportion to the length of the diffractive element set. Theoverall length of the diffractive element set maybe reduced untiladjacent diffracted spectral peaks overlap, or become comparable to orgreater than the overall bandpass Δλ. The individual mode-to-modereflective bands would then substantially continuously span a spectralprofile that encompasses the overall diffracted bandwidth givenapproximately by Eqs. 2 and 3. Shortening the diffractive element setwill, however, also weaken the reflection strength of the diffractiveelement set and therefore increase insertion loss. Increasing thediffractive strength of individual diffractive elements may at leastpartly compensate for this.

[0031] In some cases, a multimode planar waveguide spectral filterhaving a reflection spectrum consisting of a series of adjacentdiffracted peaks may provide useful functionality. For example, a WDMsystem based on multiplexing spectral slices of light-emitting diode(LED) sources may be implemented and not depend critically on thedetailed passband properties of the multiplexer/demultiplexer(mux/demux) employed. Such an LED spectral slicing WDM device mayinstead depend primarily on the central reflective channel wavelengthsand channel widths (Eq. 3). Using matched mux and demux multimodespectral filters having the same reflective passband structure will tendto minimize net insertion loss. It should be noted that multi-modeplanar waveguide devices are well suited to work with LED sources sincethe multimode planar waveguide readily accepts light in many transverseinput optical modes and a continuum of in-plane optical modes.

[0032] The discrete narrow diffracted spectral peaks characteristic of afixed-optical-spacing multimode spectral filter may be shifted so thatthe diffractive element set provides a smooth continuous reflectiveprofile throughout the fractional bandwidth given by Eq. 3. For example,the diffractive element set may be formed so that the optical spacingbetween successive diffractive elements is chirped along the set. Localquantities Λ_(loc), Δλ_(loc), and λ_(loc), characterizing localizedportions of the diffractive element set, would be used in Equations (1),(2), and (3). The chirp may be sufficiently strong so that themode-to-mode diffracted spectral peaks are shifted by an amount at leastas large as the maximum local bandpass Δλ_(loc) for the diffractiveelement set. As a result, the overall diffractive spectral profile ofthe multimode spectral filter would be substantially continuous, andwould be at least as spectrally broad as the larger of the broadestmode-to-mode peak or 2Δλ_(loc). Instead of a monotonic variation ofΛ_(loc), Δλ_(loc), and λ_(loc) along the diffractive element set (i.e.,chirp), the optical spacing of the diffractive elements may be varied inmany ways so as to provide a single smooth reflection spectrum for thediffractive element set. For example, the diffractive element set couldbe divided into discrete segments, each having constant values ofΛ_(loc), Δλ_(loc), and λ_(loc) that differ from other segments of theset. The variations in Λ_(loc), Δλ_(loc), and λ_(loc) may be achieved ina variety of ways. The physical spacing (i.e., ray path increment) ofthe diffractive elements may be spatially varied within a set (chirped,segmented, or otherwise). This may be achieved by suitable adaptation ofthe spatially selective fabrication of the diffractive element set, forexample, by suitable modification of a lithographic mask or a stampingpattern. Alternatively, the effective modal indices may be spatiallyvaried within a set of diffractive elements, thereby altering Λ_(loc),Δλ_(loc), and λ_(loc) for a fixed physical spacing of the diffractiveelements. This may be accomplished in a variety of ways, for example, byspatially varying exposure of sensitive waveguide material toultraviolet radiation.

[0033] In order to make a multiplexer or demultiplexer using sets ofdiffractive elements, individual diffractive element sets may beoverlapped, interleaved, overlaid, and/or positioned sequentially in/onthe planar waveguide. For a demux, polychromatic light enters a singleinput optical port. Multiple overlapped, interleaved, overlaid, orsequential diffractive element sets are employed, each of which has adiffractive element optical spacing (constant or otherwise) set to bereflective within one (or more) spectral channels, and each of whichroutes reflected light from the input optical port to one of multipleoutput optical ports. The multiple diffractive element sets collectivelyroute light from the input optical port to the multiple output opticalports with a predetermined mapping of input spectral channels to outputoptical ports—each diffractive element set handles the mapping of one ormore spectral channels to a specific output optical port. A multiplexeroperates in a reverse manner from the demultiplexer.

[0034] The curvilinear shape of the diffractive element contours may bedetermined by a variety of standard optical imaging system design tools.Essentially, each diffractive element contour may be optimized to imagethe input port onto the output port in a phase coherent manner. Inputsto the design are the detailed structure of the optical input and outputports and their locations. Standard ray tracing approaches to opticalelement design may provide a diffractive contour at each opticaldistance into the planar waveguide that will provide an optimal imagingof the input signal at the input port onto the optimal output signal atthe output port. Simple curves may be employed as approximations of thefully optimized contours. Diffractive contours are spaced by an opticalpath difference (as described above) that provides for the field imageof successive diffractive contours to be substantially in phase at adesired wavelength. If the overall response of the diffractive elementset is to be apodized with phase modulation, the optical spacing ofsuccessive diffractive element contours may be controlled to providerequired phase differences between diffracted components at the outputport.

[0035] An alternative approach to designing the diffractive elementcontours for a diffractive element set is to calculate interferencepatterns between simulated fields at a desired wavelength entering theinput port and exiting the output port in specific planar waveguideoptical transverse modes. One approach is to consider the interferencepattern of each input mode having a desired optical field variation inthe plane of the slab waveguide and calculating the interference patternwith a field coupled by the diffractive element set to a desired outputoptical field variation in the plane of the slab at the output port. Thenet pattern to be formed or written as a set of diffractive elementswould be the sum of the individual calculated mode-to-mode interferencepatterns. In forming or writing a summed pattern for the diffractiveelement set, suitable discretization is applied as needed for anylithographic or UV exposure approach that is utilized for fabrication.It is also possible to form or write the diffractive element set basedon a single input-mode-to-output-mode interference pattern topreferentially couple certain modes through the planar waveguide.

[0036] In an alternative method for making the diffractive elementstructure, the core consists of a material of appropriate index that isalso photosensitive at the wavelength of the desired operational signalbeams. As in traditional holography, the input and output recordingbeams (same wavelength as operational signal beams of the envisioneddevice) are overlapped in the core and the interference pattern betweenthem is recorded. Subsequently the core material is developed and, ifnecessary, a cladding may be deposited or attached by other means.

[0037] The holographic structure may also be designed by interference ofcomputer-generated beams. Unlike in single mode waveguides, theresulting interference pattern extends in all three spatial dimensions.Reduction to a planar slab waveguide may occur by slicing through anappropriate horizontal cross section of the interference pattern.Alternatively, the three dimensional hologram can be built up andapproximated by combining several of the discrete cross-sections andthen stacking them up. This may be implemented in several lithographicsteps.

[0038] It should be noted that many of the embodiments depicted in thisdisclosure are only shown schematically, and that not all the featuresmay be shown in full detail or in proper proportion and/or location.Certain features or structures may be exaggerated relative to others forclarity. In particular, it should be noted that the numbers ofdiffractive elements in an actual device may typically be larger thanthat shown in the Figures. The numbers of diffractive elements isreduced in the Figures for clarity. It should be further noted that theembodiments shown in the Figures are exemplary only, and should not beconstrued as specifically limiting the scope of the written descriptionor the claims set forth herein. It is intended that equivalents of thedisclosed exemplary embodiments and methods shall fall within the scopeof the present disclosure. It is intended that the disclosed exemplaryembodiments and methods, and equivalents thereof, may be modified whileremaining within the scope of the present disclosure.

What is claimed is:
 1. An optical apparatus, comprising a planar opticalwaveguide having at least one set of diffractive elements, the planaroptical waveguide substantially confining in one transverse spatialdimension an optical signal propagating in two other spatial dimensionstherein, wherein: the planar waveguide supports multiple opticaltransverse modes in the confined transverse dimension; each diffractiveelement set routes, between a corresponding input optical port and acorresponding output optical port, a corresponding diffracted portion ofthe optical signal propagating in the planar waveguide that isdiffracted by the diffractive element set; the optical signal issuccessively incident on the diffractive elements; and the correspondingdiffracted portion of the optical signal reaches the correspondingoutput optical port as a superposition of multiple optical transversemodes supported by the planar optical waveguide.
 2. The apparatus ofclaim 1, further comprising at least one corresponding multimode opticalsource positioned and aligned so as to launch the optical signal intothe planar waveguide through the corresponding input optical port as asuperposition of multiple optical transverse modes supported by theplanar optical waveguide.
 3. The apparatus of claim 2, wherein themultimode optical source comprises a multimode channel opticalwaveguide.
 4. The apparatus of claim 2, wherein the multimode opticalsource comprises a multimode optical fiber.
 5. The apparatus of claim 2,wherein the multimode optical source comprises a light-emitting diode.6. The apparatus of claim 2, wherein the multimode optical sourcecomprises a laser diode.
 7. The apparatus of claim 1, further comprisingat least one corresponding multimode output optical waveguide positionedand aligned so as to receive the corresponding diffracted portion of theoptical signal through the corresponding output optical port.
 8. Theapparatus of claim 7, wherein the multimode output optical waveguidecomprises a multimode channel optical waveguide.
 9. The apparatus ofclaim 7, wherein the multimode output optical waveguide comprises amultimode optical fiber.
 10. The apparatus of claim 1, furthercomprising at least one corresponding photodetector positioned andaligned at the corresponding output optical port so as to receive thecorresponding diffracted portion of the optical signal through thecorresponding output optical port.
 11. The apparatus of claim 1, whereinthe diffractive elements are curvilinear diffractive elements.
 12. Theapparatus of claim 11, wherein the diffractive elements are arranged asfocusing elements, and the corresponding input optical port and thecorresponding output optical port are located at corresponding conjugateimage points defined by the focusing elements.
 13. The apparatus ofclaim 12, wherein the focusing elements define a non-unity conjugateratio.
 14. The apparatus of claim 11, wherein the diffractive elementsare arranged so as to yield a substantially collimated diffractedportion of the optical signal.
 15. The apparatus of claim 1, wherein:the planar optical waveguide comprises a core surrounded by first andsecond claddings, a core index n_(core) being larger than respectivecladding indices n₁ and n₂; each localized portion of each diffractiveelement set diffracts a corresponding portion of the output signal thatfalls within a set of mode-to-mode diffracted spectral peaks, whereincenter wavelengths of the mode-to-mode diffracted spectral peaks fallwithin a corresponding diffractive element set local bandpass Δλ_(loc)at a corresponding diffractive element set nominal local wavelengthλ_(loc), where λ_(loc) is twice a local optical path difference, withinthe planar optical waveguide between adjacent diffractive elements,divided by a selected diffracted order; and a diffractive element setlocal fractional bandpass ratio Δλ_(loc)/λ_(loc) is substantially equalto (n_(core)−n_(clad))/n_(core), where n_(clad) is the larger of n₁ andn₂.
 16. The apparatus of claim 15, wherein λ_(loc) is substantiallyspatially uniform along at least one of the diffractive element sets.17. The apparatus of claim 16, wherein the diffractive element set withsubstantially spatially uniform λ_(loc) is arranged so that themode-to-mode spectral peaks are sufficiently spectrally broad so as tooverlap, resulting in substantially continuous diffraction of theoptical signal across the bandpass Δλ_(loc).
 18. The apparatus of claim15, wherein λ_(loc) spatially varies along at least one of thediffractive element sets.
 19. The apparatus of claim 18, wherein λ_(loc)is spatially chirped along at least one of the diffractive element sets.20. The apparatus of claim 18, wherein λ_(loc) is spatially segmentedalong at least one of the diffractive element sets.
 21. The apparatus ofclaim 18, wherein spectral shifting of individual mode-to-modediffracted spectral peaks due to the spatial variation of λ_(loc)results in substantially continuous diffraction of the optical signalacross a bandpass Δλ_(set), where Δλ_(set) is about equal to Δλ_(loc)plus the range of variation of λ_(loc) along the diffractive elementset.
 22. The apparatus of claim 1, wherein the planar optical waveguidecomprises a core surrounded by lower-index cladding, and the diffractiveelements are located at an interface between the core and cladding. 23.The apparatus of claim 1, wherein the planar optical waveguide comprisesa core surrounded by lower-index cladding, and the diffractive elementsare located entirely within the core.
 24. The apparatus of claim 1,wherein the planar optical waveguide comprises a core surrounded bylower-index cladding, and the diffractive elements span the entirethickness of the core.
 25. The apparatus of claim 1, wherein the planaroptical waveguide comprises a core surrounded by lower-index cladding,and the diffractive elements are located in the cladding.
 26. Theapparatus of claim 1, wherein: the optical element has multiple sets ofdiffractive elements; each diffractive element set routes, between acorresponding input optical port and a corresponding output opticalport, a corresponding portion of the optical signal that is diffractedby the diffractive element set; and each corresponding portion of theoptical signal reaches the corresponding output optical port as asuperposition of multiple optical transverse modes supported by theplanar optical waveguide.
 27. The apparatus of claim 26, wherein atleast two of the corresponding diffracted portions of the optical signalfall within distinct spectral passbands.
 28. The apparatus of claim 26,wherein at least two of the multiple diffractive element sets areoverlaid.
 29. The apparatus of claim 26, wherein at least two of themultiple diffractive element sets are positioned sequentially.
 30. Theapparatus of claim 26, wherein at least two of the multiple diffractiveelement sets are interleaved.
 31. The apparatus of claim 26, furthercomprising multiple optical input ports, wherein at least two diffractedportions of the optical signal are routed from different ones of themultiple optical input ports.
 32. The apparatus of claim 26, furthercomprising multiple optical output ports, wherein at least twodiffracted portions of the optical signal are routed to different onesof the multiple optical output ports.
 33. A method, comprising:launching an optical signal through an input optical port into a planarwaveguide, the planar optical waveguide substantially confining in onetransverse dimension the optical signal propagating in two otherdimensions therein; and receiving from the planar optical waveguidethrough an output optical port at least one diffracted portion of theoptical signal diffracted by a corresponding one of at least onediffractive element set of the planar waveguide, wherein: the planarwaveguide supports multiple optical transverse modes in the confinedtransverse dimension; each diffractive element set routes, between acorresponding input optical port and a corresponding output opticalport, the corresponding diffracted portion of the optical signalpropagating in the planar waveguide that is diffracted by thediffractive element set; and the corresponding diffracted portion of theoptical signal received through the output optical port reaches theoutput optical port as a superposition of multiple transverse opticalmodes supported by the planar optical waveguide.
 34. The method of claim33, wherein at least one corresponding multimode optical source ispositioned and aligned so as to launch the optical signal into theplanar waveguide through the corresponding input optical port as asuperposition of multiple optical transverse modes supported by theplanar optical waveguide.
 35. The method of claim 34, wherein themultimode optical source comprises a multimode channel opticalwaveguide.
 36. The method of claim 34, wherein the multimode opticalsource comprises a multimode optical fiber.
 37. The method of claim 34,wherein the multimode optical source comprises a light-emitting diode.38. The method of claim 34, wherein the multimode optical sourcecomprises a laser diode.
 39. The method of claim 33, wherein at leastone corresponding multimode output optical waveguide is positioned andaligned so as to receive the corresponding diffracted portion of theoptical signal through the corresponding output optical port.
 40. Themethod of claim 39, wherein the multimode output optical waveguidecomprises a multimode channel optical waveguide.
 41. The method of claim39, wherein the multimode output optical waveguide comprises a multimodeoptical fiber.
 42. The method of claim 33, wherein at least onecorresponding photodetector is positioned and aligned at thecorresponding output optical port so as to receive the correspondingdiffracted portion of the optical signal through the correspondingoutput optical port.
 43. The method of claim 33, wherein the diffractiveelements are curvilinear diffractive elements.
 44. The method of claim43, wherein the diffractive elements are arranged as focusing elements,and the corresponding input optical port and the corresponding outputoptical port are located at corresponding conjugate image points definedby the focusing elements.
 45. The method of claim 44, wherein thefocusing elements define a non-unity conjugate ratio.
 46. The method ofclaim 43, wherein the diffractive elements are arranged so as to yield asubstantially collimated diffracted portion of the optical signal. 47.The method of claim 33, wherein: the planar optical waveguide comprisesa core surrounded by first and second claddings, a core index n_(core)being larger than respective cladding indices n₁ and n₂; each localizedportion of each diffractive element set diffracts a correspondingportion of the output signal that falls within a set of mode-to-modediffracted spectral peaks, wherein center wavelengths of themode-to-mode diffracted spectral peaks fall within a correspondingdiffractive element set local bandpass Δλ_(loc) at a correspondingdiffractive element set nominal local wavelength λ_(loc), where λ_(loc)is twice a local optical path difference, within the planar opticalwaveguide between adjacent diffractive elements, divided by a selecteddiffracted order; and a diffractive element set local fractionalbandpass ratio Δλ_(loc)/λ_(loc) is substantially equal to(n_(core)−n_(clad))/n_(core), where n_(clad) is the larger of n₁ and n₂.48. The method of claim 47, wherein λ_(loc) is substantially spatiallyuniform along at least one of the diffractive element sets.
 49. Themethod of claim 49, wherein the diffractive element set withsubstantially spatially uniform λ_(loc) is arranged so that themode-to-mode spectral peaks are sufficiently spectrally broad so as tooverlap, resulting in substantially continuous diffraction of theoptical signal across the bandpass Δλ_(loc).
 50. The method of claim 47,wherein λ_(loc) spatially varies along at least one of the diffractiveelement sets.
 51. The method of claim 50, wherein λ_(loc) is spatiallychirped along at least one of the diffractive element sets.
 52. Themethod of claim 50, wherein λ_(loc) is spatially segmented along atleast one of the diffractive element sets.
 53. The method of claim 50,wherein spectral shifting of individual mode-to-mode diffracted spectralpeaks due to the spatial variation of λ_(loc) results in substantiallycontinuous diffraction of the optical signal across a bandpass Δλ_(set),where Δλ_(set) is about equal to Δλ_(loc) plus the range of variation ofλ_(loc) along the diffractive element set.
 54. The method of claim 33,wherein the planar optical waveguide comprises a core surrounded bylower-index cladding, and the diffractive elements are located at aninterface between the core and cladding.
 55. The method of claim 33,wherein the planar optical waveguide comprises a core surrounded bylower-index cladding, and the diffractive elements are located entirelywithin the core.
 56. The method of claim 33, wherein the planar opticalwaveguide comprises a core surrounded by lower-index cladding, and thediffractive elements span the entire thickness of the core.
 57. Themethod of claim 33, wherein the planar optical waveguide comprises acore surrounded by lower-index cladding, and the diffractive elementsare located in the cladding.
 58. The method of claim 33, wherein: theoptical element has multiple sets of diffractive elements; eachdiffractive element set routes, between a corresponding input opticalport and a corresponding output optical port, a corresponding portion ofthe optical signal that is diffracted by the diffractive element set;and each corresponding portion of the optical signal reaches thecorresponding output optical port as a superposition of multiple opticaltransverse modes supported by the planar optical waveguide.
 59. Themethod of claim 58, wherein at least two of the corresponding diffractedportions of the optical signal fall within distinct spectral passbands.60. The method of claim 58, wherein at least two of the multiplediffractive element sets are overlaid.
 61. The method of claim 58,wherein at least two of the multiple diffractive element sets arepositioned sequentially.
 62. The method of claim 58, wherein at leasttwo of the multiple diffractive element sets are interleaved.
 63. Themethod of claim 58, wherein the planar waveguide includes multipleoptical input ports, wherein at least two diffracted portions of theoptical signal are routed from different ones of the multiple opticalinput ports.
 64. The method of claim 58, wherein the planar waveguideincludes multiple optical output ports, wherein at least two diffractedportions of the optical signal are routed to different ones of themultiple optical output ports.